MILK SCIENCE
Sometimes, it might seem like we sell more milk than coffee. In a sense that’s true; about 80% of our most popular drinks is milk. Good milk can make or break a delicious latte - but what makes for good milk? Why is alternative milk so difficult to steam? Let’s take a look at the science of good milk foam, and how to best achieve it.
Milk is around 87% water - the remaining 13% is what gives milk its flavor and unique qualities. Of that 13%: 27% is lactose (milk sugar) 31% is fat, 26% is protein, and 6% is made up of various vitamins and minerals - everything a baby cow needs to grow big and strong. As coffee professionals, we’re mainly concerned with lactose, fat, and protein - especially protein. To achieve this makeup a balance of nutrients is needed in the cow’s diet. Farmers usually provide three sources of food, often year-round: grass or hay from the pasture, silage (a fermented, high-moisture feed; think sauerkraut for cows), and cake (a supplemental protein source made from cereals and soy). Cattle are usually kept out of the pasture during the winter months, when their hooves can churn up the wet ground. This is important for us because it means that there IS a difference between summer and winter milk! You’ll likely notice a difference as the weather begins to turn.
When it comes to steaming milk or creating microfoam, a couple of things are happening. The act of steaming milk itself is a two step process. First we “entrain” air into the milk. It introduces air bubbles into the liquid of the milk, thereby texturizing it and creating a silky mouthfeel. This also increases the volume of the milk by about 33%. Second, we are heating the milk to about 155*F for ideal drink making. This also helps keep microfilm stable.
Several things also happen chemically to the milk as we introduce air and heat it - specifically two that are important to us as baristas. Milk sugar (lactose) is a “disaccharide,” meaning it is made up of two sugar molecules: glucose and galactose. When we heat milk past 110*F, lactose splits into its constituent monosaccharide molecules. This is important - because it’s easier for your palate to break down simpler sugars. Steamed milk will taste sweeter than non-steamed, even though no additional sugars are created. The second involves the most important component of milk for microfoam: protein. Protein molecules naturally exist as tightly coiled strands. One side of the molecule is hydrophobic, meaning it repels water; one side is hyrdophilic, meaning it attracts water. As milk is heated past 110* these molecules begin to unspool and form a grid-like structure. The hydrophobic part of the strand wraps around the microfoam bubbles, while the hydrophilic part draws in the water close around the bubbles. This is what creates luxurious, voluptuous microfoam for the best drinks.
Milk foam, courtesy of Barista Hustle. The white line is 1mm.
(It’s also why it’s so hard to steam oat and macadamia milks; they have almost no protein compared to whole milk.)
Heating milk too hot (above 165*F) will cause the protein molecules to “denature,” or fall apart. Practically speaking, this is the upper limit for how hot we can steam cow milk because in addition to tasting burnt or scalded, any microfoam you create will not hold up.
This is best summarized by Professor Steven Abbot’s Five Foam Factors:
Wet - having a high liquid content with a relatively low aeration percentage: 33% is ideal.
Fine bubbles with a small diameter - this creates lower surface tension.
Stability - milk foam above 98*F will remain relatively stable.
Moderate viscosity - un-homogenized whole milk is best for a luxurious mouthfeel.
Low yield stress - it takes little effort to disrupt the structure; pour within 10s of steaming for the best results.
Good cows produce good milk, and good milk leads to good latte art and good tasting drinks. And that’s Good For The People.